We will develop a fast, low-dose, direct-conversion x-ray detector for pre-clinical small animal imaging. The detector will improve micro-CT imaging of animal and specimen with fast readout, reduced dose, high throughput, and low detector cost. It can be incorporated into micro-CT systems to reduce the radiation dose delivered to animals and will enable new procedures that are not currently feasible due to the characteristics of present x-ray detectors. Micro-CT scanners have become integral to the study of disease progression and to drug development in small animals. By combining the ease of animal studies with the high resolution of small-animal CT, research is faster and at a lower cost than with conventional techniques. Manufacturers of functional imaging systems such as microPET and microSPECT (1) are also integrating micro-CT detectors into their systems for localizing functional activity anatomically, performing attenuation correction, and for enabling CT-based diagnostics. Although these systems enable longitudinal studies, multiple micro-CT scans deliver a large radiation dose to the animal, which can perturb the intended experiment or even kill the subject. The dose efficiency of current scintillator-CCD or -CMOS photodiode array x-ray detectors is compromised by inefficient conversion of x-rays to light, transport of the light to the detector, and conversion of the light to electric signal. We have successfully developed a new direct x-ray converter material, polycrystalline mercuric iodide, which is a high-Z large bandgap compound semiconductor with outstanding charge collection properties. The proposed detector is a thin film of poly-crystalline mercuric iodide for direct conversion of incident x-rays into charge, coupled to a CMOS readout structure for charge readout. Dose reduction and improved throughput will be achieved due to the very high sensitivity of mercuric iodide films. We expect to widely market the device as an OEM component to scanner manufacturers. In Phase I of the project, we will develop wafer-level processing to adapt an existing CMOS readout device for compatibility with the Hgl2 film, the film deposition techniques, and final assembly testing. In Phase II we will customize the design of the CMOS readout chip for full optimization, modify the Hgl2 deposition as required, and complete a product prototype. Phase II testing of detector prototypes will be performed at both Photon Imaging and UCSF in their new small animal CT/SPECT systems.